While various sensors are used in an automation line of a factory for process control or the like, a photoelectric switch providing a photoelectric sensor with a switching function is employed for various uses since the same has such an advantage that detection of an object can be performed in a non-contact manner.
A reflection type conventional photoelectric switch is adapted to project light from a projector toward a detecting position and detect presence/absence of reflected light from the detecting position by a photoreceptor, thereby recognizing whether or not a detection object is present on the detecting position. On the other hand, a transmission type switch oppositely arranges a projector and a photoreceptor and utilizes the fact that no light reaches the photoreceptor by shading by a detection object if the same is present in a space therebetween, and is formed as the so-called photointerruptor. In either case of these, it performs opening/closing of a required switch in response to a photoreceiving state of the photoreceptor, thereby serving a function as a photoelectric switch.
However, since such a conventional photoelectric switch is on condition of light reflectivity or shadingness of the object, as to a detection object which is poor in such an optical property, particularly a transparent or semitransparent liquid or solid (hereinafter "substance having transparency") its detection is difficult. Namely, since the light from the projector is hardly reflected or shaded in case of such a substance having transparency, a photoreceiving output of the photoreceptor hardly changes by presence/absence of the detection object in the conventional photoelectric switch. Further, while a threshold level for discriminating the photoreceiving output of the photoreceptor must be set in the vicinity of a zero level in order to forcibly detect slight reflection or shading, it comes to that switching of ON/OFF takes place only by small disturbance such as noise when doing so, and this causes wrong information.
On the other hand, every substance is not completely transparent with respect to all wavelengths, but has light absorbance depending on the wavelength of light, as is generally known. In correspondence to this, there is proposed a technique of extracting a specific wavelength by passing light from a white light source through a filter, projecting it to an object, and performing presence/absence detection of the object or the like in response to the absorbance of the light passing through the object.
However, a transmission waveband readily available as a filter does not necessarily coincide with the light absorption waveband of the object to be detected in this case. Therefore, it often happens that detection sensitivity does not reach a necessary level in practice.
Further, an incandescent lamp used as the white light source has a large size, and hence an optical system for projecting this to the object also increases in size. Therefore, the size of the projector which must contain these components increases.
Under such circumstances, a light absorption utilization type photoelectric detector employing a white light source and a filter is limited in its use, and it is difficult to employ the same for an apparatus such as a photoelectric switch which is miniature and used generically.
Further, it is difficult to obtain a complete filter transmitting light of only a noted wavelength by such a technique, and light other than the wavelength is considerably transmitted through the filter in the actual situation. Therefore, it is difficult to project only light of a specific wavelength responsive to absorptivity of the object, and it often happens that a number of wavelengths are mixed in the detection. Thus, influence of absorption as to other than the noted wavelength is unavoidable, and there are some cases that detection of the object becomes incorrect.
Thus, detection of a detection object which is poor in optical property with light in a non-contact manner has various problems, while there is also a problem that a larger number of other sensors must be mounted in problem order to mount one sensor. In particular, photoelectric switches are utilized for various uses, and hence a number of similar photoelectric switches are arranged/disposed on a single automation line in the actual situation.
Even for a detecting operation of merely detecting the quantity of a liquid in a container, for example, it comes to that photoelectric switches performing two types of detection as to whether or not the container has reached an injecting position and whether or not the injection quantity is proper in the automation line.
Thus, a wide installation space is necessary for a number of photoelectric switches in the actual automation line, and much time is required for installing these many photoelectric switches in case of manufacturing the automation line itself.
Incidentally, the automation line of a factory is shifting to multi-type small-lot production in recent years, such that products having various shapes, colors and the like generally flow on the same line. In order to cope with such circumstances, it has become necessary to install a larger number of sensors responsive to conditions on the line with respect to products of various different conditions despite the same or similar objects. This is because the conventional photoelectric switch has been so designed that the same can perform only one type detection under one condition and hence various types of photoelectric switches responsive to various detection objects are required.
As hereinabove described, the photoelectric switch has such excellent utility value that the same can detect a detection object in a non-contact manner, and it can be said that enlargement of its use and convenience is important in future research and development.